The present invention relates to load sensing power steering systems, and more particularly, to such systems which simultaneously provide flow to an auxiliary circuit.
It has long been an object of those working in the hydraulics art to provide flow control circuits for controlling fluid actuated devices in which it is possible to control the rate of fluid flow, and therefore, the operational speed of the fluid actuated device, independent of the varying external loads imposed upon the fluid actuated device. This has been considered especially desirable in the development of hydrostatic power steering systems utilizing a source of pressurized fluid feeding a power steering cylinder to turn a pair of ground-engaging wheels, with a steering control valve disposed between the source of fluid and the cylinder and responsive to rotation of the steering wheel to permit a controlled rate of fluid flow to the cylinder.
One of the early arrangements for making the rate of fluid flow to the fluid actuated device (the fluid motor) dependent only on the setting of the flow control valve, and not dependent on the load of the fluid motor, is illustrated in U.S. Pat. No. 2,503,870, issued to Harrington. The system of the referenced patent discloses a well-known four-way directional flow control valve disposed in series flow relationship between a fluid pump and a fluid motor. Disposed in series flow relationship between the pump and the directional flow control valve is a flow regulating valve having a valve member positioned in accordance with the pressure drop across the variable orifice formed between the pressure port and the motor port of the directional flow control valve, upon actuation thereof. The valve member in the flow regulating valve is operable to bleed off to the reservoir a portion of the incoming flow from the pump in a manner which is now well-known in the art, to maintain a relatively constant pressure drop across the variable orifice of the directional flow control valve.
A subsequent step in the development of such systems is illustrated in U.S. Pat. No. 3,445,210, issued to J. D. Allen, and assigned to the assignee of the present invention. The flow control arrangement illustrated in the Allen patent is generally similar in purpose and operation to that shown in the Harrington patent, except that the Allen arrangement uses a pressure compensating valve in place of the flow regulating valve of Harrington, and instead of bypassing excess flow to the reservoir, the pressure compensating valve of Allen makes the excess flow available for operation of an auxiliary fluid circuit. This is made possible by the fact that the valve of Allen, unlike that of Harrington, is arranged such that the movable valve member is positioned in accordance with the pressure drop across the variable orifice of the directional flow control valve, and independently of the fluid pressure in the auxiliary fluid circuit.
In the development of hydrostatic power steering systems, there have been commercially available for more than a year prior to the date of the present application, systems in which a pressure signal sensed within the steering control valve has been used to control the stroke of a variable displacement pump, with the objective of maintaining a relatively constant pressure drop across the steering control valve, or a portin thereof, during variations in steering load. As used herein, the term "steering load" is intended to means and include a fluid pressure which exerts sufficient force within the steering motor to overcome the ground forces acting on the steered wheels.
The above-referenced commercially available system is illustrated in U.S. Pat. No. 3,915,253, issued to Ott et al, and includes a steering control valve which defines a first variable orifice operable to determine the fluid flow rate through the steering control valve, a second orifice which determines the direction of fluid flow (right turn or left turn), a fluid meter, a third orifice downstream from the fluid meter, a fourth variable orifice, the steering motor, and a fifth variable orifice on the return side of the steering motor. A pressure signal is taken just downstream from the third orifice and is communicated back to the stroke control mechanism of the variable displacement pump which is also responsive to a pressure signal just downstream from the output of the pump. Therefore, the pump is being controlled, in effect, by a load signal taken across the series combination of the first and second orifice, the fluid motor, and the third orifice. The system disclosed in the Ott patent provides a relatively large pressure drop across the series combination of the previously-mentioned orifices. If desired, it is well-known to reduce this pressure drop by simply eliminating the orifices located in series with the first variable orifice.
Among the major limitations resulting from the use of the steering system of the Ott patent are the lack of a high pressure carryover capability and that the system may be used only with the type of variable displacement pump and pressure compensator valve arrangement illustrated therein. The interest in steering systems having high pressure carryover capability has been greatly increasing in the past several years, in recognition of the need to use available fuel sources more efficiently. A hydrostatic power steering system having a high pressure carryover capability makes it possible to operate both the steering motor and an auxiliary device such as a back hoe or a crane boom using only one pump, while converting a greater percentage of the input energy to that pump into useful work.
It should be apparent that a power steering system which may be used with various pump configurations is of greater commercial value solely because of the increased system versatility. For example, it is then possible to have a closed center system, an open center system, or a load sensing system. Furthermore, such a versatile power steering system is more marketable because it is more easily interchangeable with currently-used systems on existing equipment.
In the typical hydrostatic power steering systems of the type to which the present invention relates, the steering control valve is designed to operate in response to a predetermined pressure differential across the variable orifice of the steering control valve which controls the rate of fluid flow through the steering control valve, and therefore, the rate of actuation of the steering cylinder. The variable orifice in the steering control valve varies from a minimum orifice area to a maximum orifice area in response to the turning of the steering wheel, and as the orifice area increases, the flow through the orifice increases. A particular steering control system is designed to operate at a predetermined rate of fluid flow through the steering control valve for a predetermined maximum valve deflection (for example, 10.degree. of rotation of the valving). As is well-known in the art, the predetermined flow rate will occur at the maximum valve deflection only in response to a particular pressure drop across the orifice, and if the pressure drop across the variable orifice is greater than that for which the steering control valve was designed, the predetermined flow rate will occur at a smaller valve deflection than the predetermined maximum. Achieving the maximum rate of actuation contemplated by the design of the steering control valve, but a smaller valve deflection, results in an undesirable increase in the sensitivity of the steering system.
The need for a constant pressure drop across the variable orifice of the steering control valve and therefore, a constant input pressure to the steering control valve for a given steering load, represents a serious disadvantage in certain commercially available power steering systems which use a priority valve downstream of the pump, but which tee off of the line connecting the pump and priority valve to feed fluid to the steering control valve. In such an arrangement, the priority valve is not truly serving as a priority valve, but merely as a pressure compensating valve to bypass a certain portion of the pump output to the auxiliary circuit without simultaneously controlling the pressure of the fluid being fed to the steering control valve.
Another limitation on the use of load sensing power steering systems has been the effect of line losses, i.e., the effect of any pressure drops in the conduit feeding the steering control valve when the steering control valve is located remotely from the mechanism which controls the pressure of the fluid fed to the steering control valve in response to the pressure signals taken respectively upstream and downstream of the main control orifice of the steering control valve.